In order to perform communication between modules in a computer system having one or more master modules, such as a CPU, and one or more slave modules, it is necessary that the modules are connected to each other via a common interface. The communication between the modules is commonly performed via a bus.
If a protocol sequence stops because of some physical or logical error between a master module and the bus, or between the bus and a slave module, a state that causes the system to hang up. In a conventional computer system, detection of a hang-up and reactivation of the system with the use of a watchdog timer is performed.
The watchdog timer has a down counter, the value of which is decremented each time a predetermined time period elapses, and it generates a reset signal (or an interrupt signal) when the counter value becomes 0. After the watchdog timer is activated, a task for returning the counter value of the watchdog timer to an initial value is periodically executed in a normal state.
However, if a hang-up or the like occurs in the system, initialization of the counter value of the watchdog timer by the task is not performed, the counter value reaches 0, and a reset signal or an interrupt signal is outputted. In this way, the whole system is reset and initialized, or an interrupt is caused and the whole system is initialized within an interrupt handler, and then, the system is reactivated.
Japanese Laid-open Patent Publication No. 04-266112 discloses a method for, in a system configured by one master and one or more slaves, early finding of a hang-up caused because the power supply is not turned on at the time of activation of a system, by the master detecting whether the power supply for each slave is turned on or not.
Japanese Laid-open Patent Publication No. 06-236329 discloses an apparatus which, when detecting a deadlock state caused by substantially simultaneous bus control requests from multiple masters in a multiple bus system, masks one bus control request for a random period to avoid the deadlock state.